Spreads, or risk premiums, on recently produced agency mortgage bonds have widened by about 0.3 percentage point since mid-September to 1.54 percentage points over a blend of Treasury bonds. That's the widest since early July.

Bond yields, meanwhile, have been climbing in recent weeks, potentially because of markets' growing perception that Donald Trump could win, Republicans could gain control of both houses of Congress, and government borrowing might surge amid more tax cuts.

Mortgage bonds get hit hard when there's growing uncertainty about the direction of rates, because their returns depend so much on it. That path determines how likely homeowners are to refinance their debt and hand back principal to investors early. Uncertainty is high now: an index tracking interest-rate volatility has risen to close to its loftiest level in about a year.

Questions about the trajectory of interest rates might fade after the election, said John Kerschner, head of securitized products at Janus Henderson. In the meantime, mortgage-backed securities are relatively cheap, he said.

"Mortgage bonds looks very attractive versus high-grade credit currently but the timing of the trade is paramount," Kerschner said. "It could be a rocky road for the next few weeks before it pays off."

Much depends on how the U.S. elections actually turn out, which at this point is an open question. Former President Donald Trump and current Vice President Kamala Harris are locked in a dead heat. Which parties end up with control of the Congress will determine how likely either president is to enact their policies.

The worst outcome for MBS could be a Republican sweep of the presidency and Congress, said Erica Adelberg, a senior mortgage bond strategist at Bloomberg Intelligence.

"Policies such as tariffs, fiscal stimulus from tax cuts, and as a result, deficits — those are going to be much more dramatic in a Trump presidency," Adelberg, author of a recent note, said. "Because higher volatility may hurt MBS performance, MBS investors should be paying attention."

A Democratic sweep of the White House and Congress might also result in a wider deficit, but with MBS spreads having already widened, the impact would probably relatively slight, she said. The fiscal impact of Kamala Harris's policies is about half of the impact of Trump's, according to an analysis from the Committee for a Responsible Federal Budget, a nonpartisan and nonprofit group.

Some betting markets have been seeing a rising probability of Trump winning, and yields in bond markets climbing in recent weeks may also reflect that possibility. That may be one reason why volatility has risen — in turn pushing out MBS spreads to levels that now look attractive, said Clayton Triick of Angel Oak Capital Advisors.

"Many investors see the potential of Trump winning the election as rate-volatility inducing, but yields have already been driven higher while rate volatility is back to summer levels on this fear," said Triick. "We view this as majority priced in and see this as a buying opportunity for agency RMBS."

Columbia Threadneedle also sees any widening in mortgage-bond spreads as an opportunity to consider adding to positions."Market participants have been unwilling to step in given the uncertainty regarding both the election and the potential path of monetary policy rates," said Jason Callan, head of structured products at the firm. "We would be looking to add on weakness."