A blue wave is expected to sweep in Congress in the midterm elections and recent polling data shows the tide is still rising.

13 congressional districts have shifted in the Democrats favor ahead of November's midterm elections, according to the Cook Political Report, with Rep. Sean Patrick Maloney's New York district and Rep. Dave Loebsack's Iowa district moving from likely Democrat to solid Democrat.

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This is yet another sign that the blue wave could look like a tsunami in 2018.

While President Donald Trump's approval rating has risen in recent weeks, ticking up from 38 percent to 40 percent since January. Trump's popularity boost is likely due to a shift in focus from deeply unpopular Republican proposals on health care and taxes to the economy, tariffs and Stormy Daniels. That is in addition to Republicans closing in on Democrat's early lead in generic polling on the question of which party voters would support for Congress.

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Still, though, Trump's popularity is historically low at 40 percent — the lowest of any modern U.S. president — and Republicans continue to trail Democrats on the generic ballot by eight points, which the Cook Political Report says is enough to erode the GOP's advantage from favorably drawn districts and endanger their majority.

Here are some reasons Democrats could win the race for Congress:

The Enthusiasm Gap

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A new report suggests Democrats enjoy a wide voter enthusiasm gap. According to a new CNN/SSRS survey, 51 percent of Democrats and Democratic-leaning voters said they were "extremely" or "very" enthusiastic about voting in November, compared to 36 percent of Republicans and Republican-leaning voters. Young voters, Trump's weakest age group, have a significant interest in the 2018 midterm elections, and overwhelmingly disapprove of Trump.

This enthusiasm gap has been glaring in off-year and special elections all cycle, including last month in Pennsylvania's 18th Congressional District, where Democrat Conor Lamb won a special House election in a district Trump had carried by nearly 20 percentage points in 2016. Turnout was especially high in Pennsylvania's special election: about 228,000 votes had been counted. That equals 62 percent of the 370,000 votes cast for presidential candidates in the district in 2016, according to data from Daily Kos Elections and analyzed by FiveThirtyEight's Nate Silver.

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Turnout was equally high in December's special Senate election in Alabama, in which 64 percent of presidential-year turnout sent turned the Yellowhammer State blue for the first time. That was an impressive win for Democrats, though one with extenuating circumstances.

A surge of Republican retirements make the GOP map tougher

Retirements are often a strong indicator for which direction a party is heading, and so far, the field continues to go downhill for Republicans, who will be defending twice as many open seats this fall as Democrats.

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There are at least 38 districts where Republicans have announced they are retiring, running for another office, or resigning outright, including 12 at serious risk of falling to Democrats, according to the Cook Report. Only 18 Democrats are exiting, and just four represent seats at serious risk of falling to the GOP, the Cook Report said.

Pennsylvania's new court-ordered congressional map

Pennsylvania's new court-ordered congressional map was another strike for Republicans. Under the previous map, Republicans won 13 of the state's 18 congressional districts in 2016, when President Trump carried 12 of the 18 states. The new map — along with the retirements of two GOP incumbents — gives Democrats the chance to pick up four to six seats.

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“This is pretty close to a Democratic wet dream,” Christopher Nicholas, a Republican consultant based in Pennsylvania, said of the new map to Politico.

The positive news for Democrats comes as former Democratic Governor Phil Bredesen has a double-digit lead over Republican Rep. Marsha Blackburn in Tennessee — an indication that a blue wave is on its way to hit another deeply red state. Despite the positive forecast for Democrats, only two of the 50 states have held their primaries so far, guaranteeing neither party will sail smoothly across 2018, so even though the blue wave is starting to look more like a tsunami now, it may end up a ripple.