During his remarks at Cheyenne High School in North Las Vegas, the president blasted Republican Rep. Joe Heck-even leading a "heck, no" chant-aligning him with the party's presidential nominee Donald Trump, whom he endorsed then later renounced.
Clinton, too, has started placing more emphasis on supporting Democrats running for Senate or House seats, given that she will need Democratic support in Congress - where both houses now are controlled by the Republicans - to ensure that she can move forward with her agenda.
Trump must win all of the states that 2012 Republican nominee Mitt Romney carried, plus pick up several that backed President George W. Bush in 2004 but turned blue in the last two elections and supported President Barack Obama.
Joe Heck, who is running against Democrat Catherine Cortez Masto for retiring Senate Minority Leader Harry Reid's seat. The race - once considered Republicans' best chance to wrest away a Democratic seat - is an example of how Mr. Trump's erratic campaign has shaken up the political map.
In his rally Sunday, Obama declared a loss in November fatal to his presidential record.
Obama's zingers at the two GOP candidates come as the president broadens his focus beyond the White House race to include Democratic candidates for governor, Senate and House, including 30 House candidates Obama was expected to endorse.
To disavow the words of Trump one doesn't have to be a Democrat, just a decent human being, Obama said. In Las Vegas, Obama said it was too late for GOP Rep. Joe Heck of Nevada to say he could not support Trump.
"And so part of the reason Michelle is working the way she is, is because she understands, as I understand, that some more fundamental values are at stake in this election", the president said. "It has to do with our basic standards of decency ― how do we treat people". "Well, no, you just got beat", Obama said.
Obama's standing in the polls has made him a popular surrogate for congressional candidates hoping that a decisive Clinton victory will benefit them as well. Marco Rubio in the first, a reminder that Clinton's team is fighting to retake a Senate majority.
Though state lawmakers don't vote on federal policies, they play an outsize role in most states in determining the way congressional lines are redrawn every 10 years.
The president added that the strongest message that could be sent in the upcoming election was if Nancy Pelosi was once again the House speaker.
Legislatures are in charge of redistricting, which influences which party controls Congress.
Obama recalled that Issa had called his administration perhaps the most corrupt in history.
After a brief conversation, Obama boarded a helicopter en route to his first destination: a $100,000-per-person evening fundraiser in Beverly Hills on behalf of Hillary Clinton's presidential campaign.
"As far as I can tell, (Darrell) Issa's primary contribution to the United States Congress has been to obstruct and to waste taxpayer dollars on trumped-up investigations that have led nowhere", Obama said.
"I believe we're actually winning", Trump declared during a round table discussion with farmers gathered next to a local pumpkin patch.
Obama will participate in another fundraiser for Clinton on Monday in San Diego. After that event, Obama flew to Los Angeles to appear on ABC's "Jimmy Kimmel Live!"