The upcoming midterm congressional elections could matter to some asset classes, though a big market impact would come only with a major surprise, analysts at Morgan Stanley said.

With control of the House and Senate up for grabs, Wall Street will be watching how the battle for power shapes up. Current expectations are that Republicans likely will maintain control of the House while ceding the Senate to the Democrats.

That divided-government scenario, while fueling partisan rancor inside the Capitol, could yield to lesser legislative action that would force attention elsewhere particularly when it comes to stocks, a team at Morgan Stanley said in a lengthy analysis for clients.

"In 2016 and 2017, investors were well served to ignore the political volatility and focus on the fundamentals," the analysts wrote. "We think the same will be true of 2018 midterms."

Among the key areas the markets should be watching are a likely deceleration of corporate profit gains and the Federal Reserve's continuation of interest rate increases.

"Outside of trade policy, we suspect that most of the policy changes in the realm of possibility will actually not alter the course of these fundamental variables all that much, and so we will prefer to spend our time reviewing incoming data and earnings reports instead of parsing polls and sentiment on Sunday morning news shows," the note said.

That doesn't mean there won't be market changes.