Julian Zelizer is a professor of history and public affairs at Princeton University, and author, with Kevin Kruse, of the forthcoming " Fault Lines: A History of the United States Since 1974. " The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) President Donald Trump is testing the patience of the GOP. Increasingly, what he thinks is best for himself and his base -- like shutting down the federal government over his demands for a border wall -- is not necessarily what is in the long- or short-term interest of his party.

Until now, the tension between the President and the GOP hadn't reached critical mass. Most of the Republican Party has supported him through thick and thin, and Trump has taken solace in his strong approval numbers among Republican voters, even as his national approval ratings tanked

But things change in presidential politics -- and the past week has presented a potential turning point. The President is doubling down on a number of issues that are not smart political bets for the Republican Party.

Polls show that the public does not want a government shutdown over the border wall. Given that the GOP will likely be blamed for it, especially after Trump defiantly and publicly took ownership of the decision, a shutdown could easily backfire on the party.

But this is not the only source of friction between Republicans and the President. Another is Trump's positive response to the decision by a federal judge in Texas to strike down the entire Affordable Care Act (ACA) because he deemed the individual mandate unconstitutional. With many Republicans promising during the midterms to protect Americans with pre-existing conditions , the GOP will be hard-pressed to come up with an alternative health care law that can garner widespread support. But still, on the heels of the ruling, Trump tweeted : "Great news for America!"

This is a losing issue for the GOP. Protection for pre-existing conditions is extremely popular. And even in red states, Medicaid expansion, a key component of the ACA, has gained popularity . Each time Republicans have moved to directly take apart the program, they have lost.

The President has taken on these controversial issues that Republicans would prefer to avoid right as the stock market is set to finish its worst year since 2008 . Although Trump devoted a good deal of time boasting about his influence on Wall Street when the stock market surged last year -- a strategy for holding the support of the business and financial wings of the GOP, which have little interest in the rest of his chaos -- the data shows 2018 ending in bad shape. The Dow Jones, CNN's Christine Romans reminds us , is down 2.7% since the President's signature tax reform legislation passed a year ago.

Meanwhile, Republicans must grapple with the fact that the President's legal problems have exploded. Nearly all the organizations that Trump has headed, The Washington Post reports , are now under investigation. Special counsel Robert Mueller's probe has now brought down a large number of the President's top advisors and it seems that he is zeroing in on Trump himself.

Former confidantes are confessing to wrongdoing, Trump's legal advisors are scrambling to keep their stories straight on national television, and there is no stability in his inner circle. He could barely find someone to take the job of chief of staff, with several GOP leaders refusing the role before budget director Mick Mulvaney agreed to take the job. If Mueller's Russia investigation turns up any evidence that implicates Trump, the problems for the Republican Party will only continue to worsen.

With Democrats taking control of the House in January, only Republicans with their heads in the sand can't see the potential disaster they are facing due to Trump's leadership. The midterms made clear that the GOP will face very real political costs for this presidency. It is a myth that Trump is a Teflon President. Things do actually stick. In some states, like Pennsylvania , Republicans are already witnessing this firsthand after they were eviscerated in November.

The GOP is entering into "put up or shut up" time. The President is counting on its loyalty and hoping that it will ignore the red lights that are flashing in Washington. This is not irrational, given what he has experienced since day one in the Oval Office. Though some Republicans like the departing Sen. Bob Corker have made statements like "we've got to remember what the Republican Party is about," they have taken little action, indicating that the GOP is fine with the status quo.

Now comes the test for the party. If Republicans want to show they are serious about divorcing themselves from this President, they have many options on the table. Congressional Republicans can join with Democrats to block Trump on the legislative front and promote the completion of Robert Mueller's investigations.

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If they don't support the border wall, they can make that clear and Trump will likely cave and give up on the effort. Or, if Republicans want to take a stand, they can support a serious and robust primary challenge for 2020 with a candidate who doesn't support the President's brand of reactionary politics.

Over the past few weeks, Trump has done everything possible to coax Republicans into standing against him. Now we will learn if anyone in the party will step up to meet this challenge or if they will remain silent and complicit in everything that this presidency has been about.