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The midterm election is also a referendum on the president and President Donald Trump's approval rating fell to 39 percent in the latest Gallup poll. Bloomberg's poll last week showed that health care is the most important issue for voters, and they don't like the way Trump has handled it.

Democrats Monday were out pushing a new agenda, aimed at business.Called "A Better Deal," the Democratic economic platform calls for more scrutiny of big mergers and a new independent agency to tackle the high costs of prescription drugs.

Clifton said about 90 percent of the swing in House races in midterm elections can be determined by the generic ballot question. He said in 2009, the data correctly pointed to an out-of-consensus view that the Democrats would lose badly in 2010.

"This was a big move no matter what number you use. This happens to every new president. This happened to Obama. It happened to Clinton. It happened to Reagan. It didn't happen to George W. Bush because of 9/11," Clifton said.

Clifton said while Republicans have a map advantage in both the House and Senate, midterm elections tend to have lower voter turnout and are presidential referendums.

"This is a sign the Republicans need to get their act together. They need to get policy passed," he said. "They've got some real issues, the Republicans."

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Data showing Republicans could lose House majority puts pressure on Trump, GOP for tax reform - CNBC