Stocks hit new record highs Friday, but despite the seemingly unstoppable bull market, one strategist said there's a historical indicator that could signal a pause in the rally.

According to Sam Stovall, chief market strategist at CFRA, generally during midterm election years the S&P 500 Index has actually fallen during the second and third quarters of that year.

With midterm elections coming up in 2018, Stovall said that while the S&P can still rally to 2,800 by year end, it may face a bumpy road to his target.

"Will the Republicans lose control in one or two houses? What will that do for infrastructure spending [and other policies]?" he said on CNBC's "Futures Now" this week.

"So I think that there is a lot of uncertainty that could be building up into a crescendo as the year moves on," he added.

That aside, Stovall is also seeing some fundamental obstacles in the market, namely with earnings expectations. While earnings growth was one of the main drivers of the market this year, Stovall said there could be a slowdown going into 2018.

"What probably will end up supporting share prices is the fact that we really don't have enough details just yet to be able to push up earnings expectations," he explained. "Yes, sentiment has been elevated, prices have been increased, but the earnings themselves have not."

However, Stovall said he anticipates that investors could "probably see S&P 500 earnings estimates go from a little less than $145 per share to a shade above $155 per share," based on the details of the GOP's tax plan.

The S&P, Dow and Nasdaq hit a record high on Friday, with all three market indexes on track for their best year since 2013.