The stock market is facing two big uncertainties this year, longtime bull Jeremy Siegel told CNBC on Thursday.
The first is how much the Federal Reserve will raise interest rates, the Wharton School finance professor said.
In December it was expected the central bank would hike rates three times in 2018, but it is "pretty well baked in now" that unless something bad happens there will be four increases this year, Siegel told "Closing Bell."
On Wednesday, the Fed held rates steady at a target of 1.5 to 1.75 percent. The market is anticipating it will announce an increase in June.
Meanwhile, a strong first-quarter earnings season isn't pushing equities significantly higher. Siegel said that's because the market was already anticipating it.
Couple that rising rate environment with the fact that stocks already priced in those better-than-expected earnings and the market is saying, "Where do I go next?" he said.
The other big unknown for the market is the midterm elections in November. History shows the elections could send the Dow Jones industrial average tumbling.
Scott Wren, senior global equity strategist at Wells Fargo Investment Institute, told CNBC last week that since 1913, the Dow has fallen an average of 20.4 percent from the highs reached the year following a presidential election to the lows carved out in a midterm election year.
Siegel said the expectation is the Democrats are going to win the House, which is now controlled by Republicans. If that happens, there's concern about the ability to push through more of President Donald Trump's agenda. There's also the possibility of impeachment proceedings against Trump.
However, the GOP also currently controls the Senate.
"If the Republicans take the Senate, there's not going to be a conviction [on impeachment]. There is not going to be a reversal of these tax cuts. There's going to be no reversal of the Republican position," Siegel said.
"They need the presidency, the Senate and the House. That can't happen for another two and a half years. So some of those dangers are delayed," he added.
— CNBC's Fred Imbert contributed to this report.