Over the weekend, US Representative Haley Stevens, who lost the Michigan Democratic Senate primary to Abdul El-Sayed in a bitter contest, switched her to backing Republican Mike Rogers.
The race is critical to Democrats' hopes of ending the 53-47 Senate majority of Trump's Republicans in Tuesday's vote.
Trump spent more than 20 minutes discussing pipes, joints, caulking, granite resurfacing, algae and leaks at the belated reopening of the Washington Reflecting Pool on Monday.
"So we have a lot of great things planned for Washington, and I could get into the nation, but for that you don't have enough time," Trump said, also mentioning the grass at the White House, his new Marine One helipad, a golf course and a triumphal arch that faces a court challenge.
He also made a brief reference to the election.
"When you vote, remember: a party that hates America cannot be allowed to rule over the people who built our country."
Analysts favour Democrats' chances of winning a majority in the House of Representatives.
Control of the Senate is more uncertain, with analysts seeing traditionally Republican-leaning states including Texas, Kansas and Iowa as competitive and giving Democrats more possible paths to a majority.
Control of either chamber would constrain Trump's ability to pass legislation and empower Democrats to investigate his administration, while a Senate majority would also allow them to block his appointees to federal courts including the Supreme Court.
A Reuters/Ipsos poll completed on Monday found 46 per cent of registered voters said they would vote for Democrats in congressional elections, compared to 38 per cent who picked Republicans. That eight-percentage-point advantage for Democrats was narrower than the 11 points found in the same poll a week earlier.
Some 32 per cent of respondents said they approved of the job Trump was doing in the White House, unchanged from a week earlier and just above his record-low reading of 31 per cent two weeks ago.
The Trump administration has sought to intervene in election disputes in Alaska and California.
A federal judge on Sunday dismissed a US Justice Department lawsuit seeking to block certification of election results in Alaska if Republican incumbent Senator Dan S Sullivan narrowly loses.
On Monday, a US judge ordered the Trump administration to stop demanding that California election officials segregate ballots cast by people on its lists of suspected noncitizens.
Trump also filed an emergency request with the US Supreme Court on Monday to temporarily block California from counting mail-in ballots from more than 20,000 registered voters who federal officials allege are likely non-US citizens.
California said the lawsuit could disenfranchise 20,000 voters based on vague accusations and that the administration's process for identifying noncitizens was flawed and prone to error.
After Stevens' endorsement of Rogers, El-Sayed's campaign said it raised $US2.4 million ($A3.4 million) on Sunday, its largest single-day fundraising haul.
El-Sayed, an Egyptian American Muslim born in Michigan, has been targeted by Trump and other Republicans who have repeatedly questioned his loyalty to the US and baselessly called him a terrorist sympathiser whose election would invite attacks on Americans. El-Sayed has called questions about his loyalty to the US "shameful".
In Alaska, the Justice Department argued voters may be confused by the presence of a challenger named Daniel J Sullivan Jr on the same ballot.
The senator said on Sunday he would accept the results of Tuesday's election even as the Trump administration presses a criminal investigation into the issue.
with AP
