Perhaps the most endangered Senate incumbent this cycle, Mark Kirk, is technically running against two-term Democratic Rep. Tammy Duckworth for his Illinois seat – but the Republican's campaign focus has been as much about vilifying Donald Trump as it is about beating back his actual opponent.

Among elected Republicans working to put a chasm between themselves and the party’s standard-bearer, Kirk has been at the vanguard.

The one-term senator fighting to hold onto the seat once held by President Obama was the first to “un-endorse” Trump – in early June – following the nominee’s ethnically charged criticism of the judge in a Trump University court case. He’s called Trump “too bigoted” and “too racist” for the job.

And Kirk was the first GOP senator to run an anti-Trump television ad, saying “Mark Kirk bucked his party to say Donald Trump is not fit to be commander-in-chief.”

Whether the efforts are enough to retain the support of blue-state voters who elevated him to the Senate in 2000 remains to be seen.

The Duckworth campaign has questioned Kirk’s motivations for bucking Trump – claiming he only dropped his support after Duckworth called him out – and whether the anti-Trump rhetoric is doing him much good.

“There is no empirical evidence to back up any claim that Kirk is even benefiting from disassociating himself from Trump – I’d say on the contrary, Kirk is losing this race on his own merits,” Duckworth campaign spokesman Matt McGrath told Fox News.

But Kirk campaign manager Kevin Artl defended the candidate’s stance.

“Senator Kirk believes that Donald Trump does not have the temperament or judgment to be our nation’s commander-in-chief,” he said. 

The race is considered very tight. One poll released in July from Republican-aligned Basswood Research showed Kirk edging Duckworth by a little over a point. 

Kirk isn’t holding back on his criticism of the sitting president, either. In an interview published in The State Journal Register over the weekend, Kirk hit the Obama administration for the delivery of a $400 million payment to Tehran, which critics have described as “ransom” for four American prisoners. Kirk said Obama was acting like “the drug dealer-in-chief.”

Meanwhile, the senator is balancing his criticism in the campaign with highlights of his own record in office.

Kirk’s television advertisement didn’t just attack Trump; it also highlighted the moderate agenda Kirk claims to support. He backs abortion rights and tougher gun laws – as the only Republican senator who voted to prevent people on the terror watch list from purchasing a gun.

The Duckworth campaign accused Kirk of trying to “pretend” to be a Democrat.

“Senator Kirk is attempting to obscure his record in an election year and wish away his long record of being a down-the-line Republican on economic issues that matter most to Illinois families,” McGrath told Fox News. “I don’t think anyone buys that this is done out of anything more than political survival.”

But Kirk’s campaign says the veteran has been a social moderate and fiscal conservative since the day he was elected to Congress in 2000.

“His independent and thoughtful leadership in always putting Illinois families first run in stark contrast with Duckworth’s extreme partisanship and ineffectiveness,” Artl told Fox News.

Duckworth – a veteran herself, who lost her legs in Iraq – comes with star-power endorsements including those of President Obama, Vice President Biden and Democratic presidential nominee Hillary Clinton.

Despite the Duckworth campaign’s claim that Kirk is distancing himself from the Republican Party, the senator has collected endorsements from over 150 Republican leaders and activists –including 2016 GOP presidential candidate and Ohio Gov. John Kasich.

A Kasich spokesman said the governor has also raised money and campaigned for Kirk. “Kirk is important for keeping majority control of the Senate and for creating bipartisan solutions to our nation’s challenges,” he said.

Meanwhile, Trump is brushing off Kirk’s jabs at him in characteristic fashion. Ahead of a meeting with senators last month, he called the Illinois senator a “loser.”