Trump's planned blitz of Senate battleground states, including Florida, Missouri and Tennessee, follows an NBC/Marist opinion poll showing the Democratic U.S. Senate candidate in Arizona taking a 6 percentage-point lead and a Quinnipiac University Poll showing Democrat Beto O'Rourke pulling closer to Republican Senator Ted Cruz in Texas.
If they won the House and Senate it would put them in a position to oppose the president's legislative agenda.
If enough of those districts switch to Democrats, the Republican party could lose its super majority in the Legislature, meaning Republicans would be unable to override gubernatorial vetoes on a party-line vote.
According to the Miami Herald, "The rallies by the parties' figureheads come as out-of-state megadonors pump millions into the governor's race and Democrats beat the bushes to help Nelson keep up with the independently wealthy and self-funding Scott".
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Doug Rivers, chief scientist at YouGov global polling, said: "The chances of the Democrats winning all of those seem to me pretty slim". Republicans have an opportunity to pick up two open seats held by Democrats in southern and northeast Minnesota.
Republicans are focusing their efforts on conservative districts Trump won by double-digit margins in 2016, particularly in rural areas. Both the developments have given the opposition Democrats an edge over the Republican.
The National Republican Congressional Committee and Democratic Congressional Campaign Committee have spent $7.6 million and $5.7 million, respectively.
Some Republicans also acknowledged that their party's "problems right now are with suburban women, and these developments in the last week affect them more than other demographic groups", said Jason Roe, a Republican consultant.
Some of these seats are in key swing states such as Florida and Pennsylvania.