North Las Vegas, Nevada: President Barack Obama on Sunday savaged Representative Joe Heck for failing to reject Donald Trump earlier in the presidential race, seeking to tarnish Heck and other Republican candidates across the country by association with a standard-bearer he called indecent and unfit for the presidency.

Speaking at a high school here as he began a three-day campaigning and fund-raising trip, Obama portrayed Heck, who is in a competitive Senate race that could determine control of that chamber, as having helped enable Trump’s rise by endorsing his breed of divisive politics. Only now, with Trump’s campaign foundering, is Heck willing to abandon him, the president said.

“I understand Joe Heck now wishes he never said those things about Donald Trump,” Obama told several thousand people, noting that Heck had said he had “high hopes” that Trump would become president and thought he was fit to have control over the nuclear codes. “But they’re on tape, they’re on the record, and now that Trump’s poll numbers are cratering, he said, ‘I’m not supporting him’? Too late! You don’t get credit for that.”

In wading so aggressively into the Nevada race, in which Heck is vying against Catherine Cortez Masto, a Democratic former state attorney-general, to succeed Harry Reid, the retiring Senate Democratic leader, Obama was working to broaden his argument against Trump into one that can sully a large number of Republicans.

The president said they had flirted for years with a strain of bare-knuckled partisanship that appealed to people who sought to delegitimize him by falsely asserting that he was born outside the United States and was plotting to take Americans’ guns and impose martial law.

Now, he said, they should be held to account for the consequences.

“They just stood by and said nothing, and their base actually began to believe this crazy stuff,” Obama said. “Donald Trump didn’t start this. He just did what he always does, which is slap his name on it, take credit for it and promote it.”

“Now, when suddenly it’s not working,” he added, “suddenly that’s a deal breaker. Well, what took you so long? What the heck?” He went on to repeat the phrase several times.

Obama implored the crowd to turn out to vote for Hillary Clinton, or see all the progress made during his tenure go “out the window.”

“Competing for the job I currently hold, you’ve got a guy who proves himself unfit for this office every single day, every single way, and on the other side, you’ve got somebody who is as qualified as anybody who’s ever run for the presidency: Hillary Rodham Clinton,” Obama said.

But he reserved his sharpest criticism for Heck, describing him as a lackey of the billionaire Koch brothers, who have invested millions in the Nevada race.

Obama has recently made campaigning and fund-raising for Clinton a passionate focus, travelling to a competitive state at least a day or two each week.

The president had always planned to be a fixture in the final days of the campaign, hoping to maximise the chances that his legacy would live on in her governing agenda.

But in recent days, Obama has expanded that ambition, working to translate Trump’s setbacks into gains for Democrats across the country.

On Thursday, he pilloried Senator Marco Rubio for condemning his party’s nominee without withdrawing his endorsement, arguing that Rubio was being hypocritical.

The situation is different in Nevada, where Heck did drop his support for Trump after the emergence this month of a leaked recording from 11 years ago in which Trump boasted about kissing and groping women without their consent.

Heck said Trump should drop out of the race, a move that appears to have cost him support among Republican base voters. That has left him narrowly trailing Cortez Masto in polls. The race — once considered Republicans’ best chance to wrest away a Democratic seat — is an example of how Trump’s erratic campaign has shaken up the political map.

The contest has attracted vast amounts of money, with organisations backed by conservatives like the Koch brothers, as well as by liberals like Tom Steyer and George Soros, pouring in a total of more than $54 million (Dh198 million), according to the Center for Responsive Politics.

The race is expected to turn in large part on voter turnout efforts. Much will depend on Hispanics, who make up more than a quarter of the population and lean toward Democrats, but who often turn out in relatively low numbers.

Imploring Nevadans to vote early, Obama said it was important that they send a message by turning out in droves.

“Do it big — don’t leave any doubt!” he said to cheers.