Donald Trump’s effort to unseat Republican Congressman Thomas Massie has transformed Kentucky’s 4th District primary into one of the most closely watched and expensive races of the US midterm season.

Massie, a conservative known for repeatedly breaking with his own party, provoked Trump’s anger after opposing US strikes on Iran and backing efforts to release files linked to Jeffrey Epstein. Now, the seven-term congressman is facing a heavily funded challenge from Ed Gallrein, a former Navy SEAL endorsed by the president.

With more than $20m spent on the race ahead of Tuesday’s vote, the Kentucky primary is shaping up as a major test of Trump’s grip over the Republican Party, and whether Republican voters will side with the president or a legislator who has built his reputation on defying party leadership.

Kentucky is also holding primaries for the open US Senate seat being vacated by longtime Republican Senator Mitch McConnell, who has served seven terms.

On the Republican side, the crowded field includes US Representative Andy Barr, who has Trump’s endorsement, and former Kentucky Attorney General Daniel Cameron.

Democrats are also competing in the Senate race, congressional contests, state legislature elections and local races across Kentucky.

Here is what we know about the race:

When do polls open and close?

Kentucky voters will head to the polls on Tuesday, May 19, for the primary election.

Polling stations will open at 6am (10:00 GMT) local time and close at 6pm (22:00 GMT) local time. Voters who are still in line when polls close will still be allowed to cast their ballots.

Most of Kentucky is in the Eastern Time Zone, so polls there will close at 6pm ET (22:00 GMT). In the western part of the state, which follows Central Time, polls will close at 6pm local time, equivalent to 7pm ET (23:00 GMT).

What races are on the ballot in Kentucky?

In Louisville, voters will cast ballots in several high-profile local contests, including races for:

• Mayor
• Clerk
• Sheriff
• Multiple Louisville Metro Council seats

Federal races

Kentucky voters will also have primary elections for:

• The US Senate
• The US House of Representatives

State legislature races are also on the ballot:

• All 100 seats in the Kentucky House of Representatives

As of April 24, there were about 3.4 million registered voters in Kentucky, including about 1.6 million registered Republicans and about 1.4 million registered Democrats.

What are the key races to watch?

Two Republican contests are expected to draw national attention in Kentucky: the US Senate primary and the battle for the 4th Congressional District, where divisions inside the Republican Party, including over the Iran war, have become a major issue.

The fight over Kentucky’s 4th District

The race is unfolding across Kentucky’s 4th Congressional District, which stretches from the outskirts of Louisville along the Ohio River and into parts of Appalachia.

At the centre of the contest is Thomas Massie, one of the few Republicans who has continued to openly challenge Trump. Since the president’s return to power, many Republican critics have either aligned with Trump, left politics or lost primaries against Trump-backed rivals.

Massie has opposed several Trump-backed policies, criticised the Iran war and US aid to Israel, and supported efforts to limit Trump’s authority to launch military action against Iran without congressional approval.

Trump and his allies have instead backed challenger Ed Gallrein, a former Navy SEAL officer, turning the race into a broader test of Trump’s grip on the Republican Party.

The contest has also attracted heavy spending from pro-Israel groups, and is increasingly being viewed as a referendum on divisions within the Republican base over foreign intervention and loyalty to Trump.

US Senate 

Kentucky’s Republican Senate primary is also getting national attention as longtime Senator Mitch McConnell steps away from Republican leadership after nearly 17 years at the top of the Senate.

“I turned 82 last week. The end of my contributions are closer than I prefer,” McConnell said earlier this year while announcing his decision. “Father Time remains undefeated.”

The race is being closely watched because it reflects the Republican Party’s broader ideological and generational shift as Trump continues reshaping the Republican party around his “Make America Great Again” movement.

For decades, McConnell represented the traditional Republican establishment, hawkish on foreign policy and closely tied to the Reagan-Bush era of conservatism. But Trump’s rise reshaped the party around a more populist movement that increasingly sidelined establishment Republicans like McConnell.

Representative Andy Barr and former Kentucky Attorney General Daniel Cameron, who are competing for the Republican nomination in Tuesday’s primary, were both once seen as close allies of McConnell. But both have also tried to distance themselves from the veteran senator as Trump’s influence over Republican voters has grown.

Although McConnell is no longer fully aligned with today’s MAGA politics, he still maintains deep influence and a loyal following in Kentucky, forcing candidates to carefully balance appeals to Trump’s base with respect for the longtime senator.

Barr received Trump’s endorsement earlier this month, with the president calling him “a Vote we can count on because he knows what it takes to GET THINGS DONE”.

Political analyst Stephen Voss of the University of Kentucky said the candidates are walking a “razor’s edge” between Republicans still loyal to McConnell and voters frustrated with the party’s old Reagan-Bush era establishment.

Why are the Kentucky primaries important?

The Republican races are being closely watched because they reflect some of the biggest debates shaping today’s Republican Party ahead of the midterm elections, including Trump’s influence over the party, divisions over foreign policy and the future direction of the Republican Party.

Democrats enter the Kentucky Senate race as clear underdogs in a state that has not elected a Democratic US senator in decades, but several candidates argue they can still compete by focusing on economic issues, healthcare and dissatisfaction with national politics.

When are results expected?

Results from Kentucky’s primary elections are expected to begin coming in shortly after polls close on Tuesday night, although the timing will vary depending on the race and county.

In the 2024 primary, the Associated Press first reported results at 6:06pm ET (22:06 GMT), just minutes after polls closed across most of the state. The final update of the night came at 9:47pm ET (01:47 GMT on Wednesday), with more than 99.9 percent of votes counted.

Kentucky requires an automatic recount for the US Senate, US House, state General Assembly and nearly all statewide offices, if the vote margin is 0.5 percent of the total vote or less.

What do the polls say about the race?

Polling in Kentucky’s most closely watched race, the Republican primary in the 4th Congressional District, suggests a very tight contest between Massie and Gallrein.

A recent poll shared by Political Polls on May 18 suggested the race was tied, while other polling averages also show both candidates running close. However, the limited amount of district-level polling and the unpredictable nature of primary turnout make the race difficult to forecast.