Celebration of San Juan de Dios festivity in Tultepec, near Mexico City, Mexico. Ronaldo Schemidt | AFP | Getty Images

There was a lot of hand-wringing on Wall Street this month as the health-care replacement bill championed by Speaker Paul Ryan and President Donald Trump failed to even come up for a vote and stocks swooned. Scary superlatives were used in the financial media this Monday like: Dow's eight-day fall longest losing streak in eight years. But after a big comeback Tuesday, the stands just 1.5 percent from its record close. Right now, it seems like nothing can derail this rally. We talked to some investors and traders to find out what could actually take this market down.

1. Trump blows his chance at tax reform

President Donald Trump speaks to members of the media as Vice President Mike Pence looks on, in the Oval Office in Washington, D.C., on March 27, 2017. Olivier Douliery | Pool via Bloomberg | Getty Images

An Axios report that Trump would be pairing an infrastructure package with tax reform in an effort to get more Democrats on his side aided the comeback Tuesday. Some sort of tax reform bill this year is a must to keep the rally going, traders say. Goldman Sachs' David Kostin, the firm's chief U.S. equity strategist, doesn't see any plan coming together until later this year, but most likely in 2018. That leaves companies with no earnings boost from the tax cuts this year and therefore the market ends lower from here, Kostin predicts.

2. Fed hiking halts companies from buying back their own stock

Federal Reserve Board Chairwoman Janet Yellen holds a news conference following a meeting of the Federal Open Market Committee March 15, 2017 in Washington, DC. Getty Images

The Fed's vice chairman, Stanley Fischer, told CNBC he sees two more hikes this year. Low interest rates have been the fuel for this bull market because companies have used the low borrowing costs to issue tons of debt and then used those funds to buy back their own stock. The end of that shell game, along with other unforeseen complications from the Fed removing its financial crisis stimulus, could knock the bull market off course, according to investors like Peter Boockvar. "This will be the first year in this cycle that all four major central banks are pulling back in some form with their easing," said Boockvar, chief market analyst at The Lindsey Group.

3. Economic growth falls short

Trump predicts he can get GDP growth to above 4 percent annually. He's got his work cut out for him because the GDPNow model from the Atlanta Federal Reserve watched by Wall Street predicts just 1 percent growth in the first quarter. Granted, Trump and the Republican-led Congress haven't passed any stimulus yet, but even when they do, economist Stephanie Pomboy believes it will be canceled out by fiscal restraint on the state and local level. The founder of MacroMavens said this is exactly what happened in 2008. She's long gold and Treasurys.

4. Economic chaos in Europe if more countries leave the euro

French presidential candidate and head of the far-right National Front party Marine Le Pen arrives on stage to give a speech on February 5, 2017. Jeff Pachoud | AFP | Getty Images

The biggest threat to the U.S. rally may come from across the pond. The U.K. began the formal Brexit process Wednesday. The French presidential election involving a far-right candidate drawing comparisons to Trump in Marine Le Pen begins in late April. And The Wall Street Journal this week reported the Italian elections early next year could be the biggest threat to global markets with a populist movement there undermining the ECB and paving the way for that country's exit from the euro.

5. Stocks could be overvalued

Pomboy said she believes stocks are in a bubble because of a simple metric that is also said to be a favorite of Warren Buffett as well: toal stock market value to GDP. "We're right back where we were in 2000 at the peak ... I struggle with the idea that the stock market has a lot of upside room barring some spontaneous acceleration in growth," Pomboy said. The only problem with this concern is that valuation is seldom something that derails a bull market alone. Typically some unforeseen catalyst occurs first, which then exposes just how out of whack things were.