If the U.S. government shuts down this weekend, will the stock market’s nearly uninterrupted march higher also come to a close?

This question is back in the news, as only a few days remain for politicians to reach a deal to avert a shutdown, though most Wall Street analysts view a closure as unlikely.

Federal government operations are funded until Friday at midnight, and unless there is a deal to extend funding, services would partially shut down. Such an outcome would add another element of political uncertainty to a market that is already overflowing with it—most notably with Special Counsel Robert Mueller’s investigation into potential Russian meddling in the 2016 presidential election.

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Gary Cohn, the top economic adviser to President Donald Trump, on Wednesday said he expects Congress to pass a short-term spending bill to avoid a shutdown. At an Axios-sponsored event, Cohn predicted a temporary spending bill to keep the government open until early next year, when the two parties will hash out a longer-term solution.

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Last week, House Republicans unveiled a short-term spending bill that would keep the government running through Jan. 19. The proposal includes controversial offsets that Democrats have already rejected.

Thursday morning, Trump tweeted that “House Democrats want a SHUTDOWN for the holidays in order to distract from the very popular, just passed, Tax Cuts.” He urged House Republicans to pass a continuing resolution.

Earlier this year, during a different showdown, Trump tweeted that the country “needs a good ‘shutdown’ in September to fix mess!”

Regardless of what happens, investors may not need to worry too much, as past shutdowns haven’t corresponded with significant stock-market selloffs. Data shows that markets have shown modest weakness during shutdowns, with the S&P 500 SPX, -1.11% falling an average of 0.6% over the period of the closure, according to data from LPL Financial. The benchmark index was only positive in 44.4% of the 18 shutdowns going back to 1976.

Chart courtesy LPL Financial

“Volatility tends to increase around these events, but they historically have had little lasting impact on markets,” wrote Craig Holke, an investment strategy analyst at Wells Fargo Investment Institute.

Stocks have shown little concern over the prospect of a shutdown. Major indexes like the Dow Jones Industrial Average DJIA, -0.87% and the Nasdaq Composite Index COMP, -1.07% are near record levels, supported by improving economic data, growing corporate profits, and the just-passed tax bill, which will cut corporate tax rates, among other changes.

In other words, a 0.6% retreat from current levels would hardly be catastrophic for equities, as all three major indexes are within 1 percentage point of record levels. However, the particular circumstances of the current government, as well as today’s investing environment, could mean the market is more vulnerable than normal.

Equities have seen little in the way of volatility or pullbacks throughout 2017, going a historically long time without retreating even 3%. It may not take much to return volatility to markets, and with valuations stretched by many metrics, investors could be looking for an excuse to take profits.

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For one thing, the Republican party controls the White House and both houses of Congress. The last time the federal government shut down with one party in control—no fewer than five shutdowns during Democrat Jimmy Carter’s administration—the impact on markets was more severe.

In those five instances—three of which occurred in 1977, with an additional one in each 1978 and 1979—markets were weaker in four of them, including a 4.4% drop during an 11-day closure in 1979.

A shutdown’s lasting market effect also remains unclear, but recent examples support the bulls. The S&P 500 rose during the past three shutdowns, suggesting “markets look past Washington’s squabbling, regardless of the length of a shutdown,” said Ryan Detrick, senior market strategist at LPL Financial.

Detrick offered those comments earlier this year, during the previous showdown, when a shutdown was avoided.

In the most recent closure, which lasted for 16 days in 2013 as conservative senators attempted to defund then-President Obama’s health-care program, the S&P 500 gained 3.1%, the best return of any shutdown in LPL’s data set.