The options-based investor anxiety gauge, the Cboe Volatility Index .VIX, recently touched a new low for the year. At around 15, it is below its long-term median of 17.6. VIX futures also look calm, as investors remain confident strong corporate earnings will continue to support markets.
"The VIX curve is not expressing any premium for midterm elections," said Michael Purves, CEO of Tallbacken Capital Advisors.
The Democratic Party has gained a significant edge over Republicans in voter perceptions of which party has a better approach to the cost of living, a troubling sign for President Donald Trump's party, a Reuters/Ipsos poll found.
Should Democrats wrest control of the House of Representatives, the shift from one-party control of the White House and both chambers of Congress could add uncertainty to markets, Julian Emanuel, lead equity and quantitative strategist at Evercore ISI, said in a note.
"A Senate flip would magnify the dynamic," Emanuel said.
The market's fearlessness combined with an array of sentiment gauges, including crowded positioning, tight credit spreads and limited demand for protection, show the market is ill-prepared to absorb any shock.
"Turbu-lens," a UBS machine-learning framework designed to forecast market vulnerability over the next month, reached the highest level of potential market stress at the end of August.
"It's been basically screaming, extreme fragility for the last several weeks now," said Maxwell Grinacoff, head of U.S. equity derivatives research at UBS.
"If you're driving a go-kart with no seatbelt down the highway at 100 miles an hour, you may get from A to B unscathed ... but if someone hits you, that's not going to be very good," he said.
So far, stocks have been unfazed by increased geopolitical tensions and a bruising bond market selloff, but analysts worry there is little margin for error.
"The system right now is not expecting any kind of shock," said Olivier d'Assier, head of investment decision research for Asia Pacific at SimCorp.