With the Russian collusion narrative exposed as fake news, President Trump has perhaps the best opportunity of his presidency to expand his base of support. He shouldn’t squander it by overreacting to the current illegal immigration influx as he seems poised to do by threatening to close the southern border.

Without Russia collusion accusations dogging him, Trump has the potential to pick up the support of some independents and the roughly ten percent of Republicans who don’t already support him. Charles Cook, who runs the Cook Political Report, expects Trump to enjoy a 2 to 5 point favorability bump as a result.

Yet Trump will throw away this goodwill — just as he did in the aftermath of the Justice Kavanaugh nomination — if he chooses to again veer off course to follow the anti-immigration siren song.

“If we don’t make a deal with Congress, the border’s going to be closed,” Trump told reporters on Tuesday. “100 percent.” Over the past week, Trump has promised to close the border several times.

Such a move would hurt the economy — something that even Trump admits — and alienate exactly the type of independent and suburban voters Republicans need to win in 2020.

Shutting down border traffic would cost the economy more than $1 billion a day — over 1.5 percent of U.S. GDP. Tens of thousands of small business owners — a traditionally Republican demographic — export hundreds of millions of dollars worth of products to Mexico each day. According to the Chamber of Commerce, “nearly half a million people legally cross the southern border every day as workers, students, shoppers, and tourists.”

Closing the southern border would disproportionately impact border states like Arizona and Texas which will be competitive in 2020. Same goes for the several southern California House districts that Republicans lost in 2018 that could flip back in 2020 if Bernie Sanders is on the ballot.

Supporters of the president’s border threat dismissively summarize the economic harm as just raising the price of guacamole. But Republicans need every avocado toast eater they can get.

The kicker: Closing the border to legal traffic would do little-to-nothing to stem the flow of illegal migrants. Even Newt Gingrich, who seemingly agrees with every Trump policy, calls the proposal “absurd.”

Trump should have already learned his lesson about squandering political opportunity by overreacting on immigration. In the wake of the Kavanaugh confirmation battle, Republicans gained a noticeable bounce in the polls. President Trump achieved one of his highest favorability ratings according to the RealClearPolitics polling average. I had several libertarian and centrist friends, who disagree with some Republican policies, tell me they would never vote Democrat again after witnessing their behavior surrounding the confirmation hearings.

Yet rather than reach out to these “sometimes Trumpers” to expand the Republican base, Trump instead went full anti-immigrant. In the span of a couple of weeks, he called up the military to respond to a caravan of a few thousand desperate migrants; promised to sign an executive order to overturn birthright citizenship in violation of the Constitution; and reiterated his support for fringe legislation to cut legal immigration levels in half.

The result? Republicans suffered their biggest midterm loss since Watergate, losing 40 seats, most of which were in traditionally Republican suburban districts. Republicans not only lost the speaker’s gavel but also future party leaders such as Mia Love, Mimi Walters, and Carlos Curbelo.

These lost districts are more immigrant, more diverse, more college educated, faster growing, and younger than the rest of the country. They are districts that should be receptive to a Republican message of economic growth, tax cuts, and opportunity for all.

To win these suburban districts, which will increasingly determine statewide and national elections, Trump and Republicans must offer a uniting pro-growth message rather than a divisive anti-immigrant one.

The conclusion of the Mueller report has given Trump and Republicans a second serve at expanding their support above 50 percent of the electorate. Closing the southern border would be a double fault.