WASHINGTON — Premiums for midlevel health plans under the Affordable Care Act will increase by an average of 25 percent next year, while consumers in some states will find significantly fewer insurance companies offering coverage, the federal government said Monday.

But the Obama administration said three-fourths of consumers would still be able to find plans for less than $100 a month with the help of federal subsidies.

The open enrollment period under President Obama’s signature health law begins on Nov. 1, but consumers got their first look at their options on Monday. Consumers who go without insurance next year could face possible tax penalties of $700 a person or more.

In many parts of the country, the available options are sure to become part of the political conversation in the election season’s closing days. And the rising costs and shrinking options all but ensure that the next president will need to make significant adjustments to the health law, something both Hillary Clinton and Donald J. Trump have promised.