For millions of Americans, Monday will mark an important deadline. In order to be covered by January 1 under a health insurance plan through Healthcare.gov , the Federal Insurance Marketplace, one must enroll by midnight on Monday, December 15.

What a difference a little over a year has made. Not too long ago, those who did not have access to affordable health insurance through a parent or employer were often faced with a choice between paying expensive premiums for coverage or putting food on the table and a keeping a roof overhead. Many opted not to purchase coverage, placing themselves and their families at great risk if they became sick or were injured. Thankfully, millions now have access to a range of affordable plans -- and tax credits to help them afford coverage -- through the state and federal health insurance marketplaces that were created under the Affordable Care Act and opened for enrollment last year. In fact, after the credits, eight out of ten current enrollees can get coverage for less than $100 a month in 2015.

Open-enrollment season for 2015 began in November, and more than a million Americans submitted applications for coverage in the first week alone. Americans across the country have been able to successfully visit Healthcare.gov to browse coverage options, find out if they are eligible for cost-saving credits and subsidies, and select the best option for them. Through Healthcare.gov consumers may also check eligibility to be covered under expanded Medicaid. Those who purchased insurance through the Marketplace last year ought to log on to see if there are more affordable plans that better meet their needs -- an estimated 70 percent of current enrollees can find a more affordable plan by returning to the Marketplace to shop again this year.

I encourage Americans in need of coverage to visit Healthcare.gov to browse a broad range of plans with different levels of coverage and affordable premiums. For those worried about privacy, plans can be browsed anonymously without having to enter any personal information, and one can always reach the Marketplace Call Center at (800) 318-2596 to sign up for coverage by phone. Signing up is easy and can mean the difference between living with the risk of needing care while uninsured or with the peace of mind that comes from having quality, affordable coverage. While Monday is the cutoff for starting the new year with coverage, the open-enrollment season will continue through February 15 for coverage beginning in February or March.

But the benefits of the Affordable Care Act are not limited to those who are finding affordable health coverage for the first time; it protects all Americans against discrimination and abuse. Among its many patient protections is a ban against insurance companies dropping coverage when consumers get sick -- and need it most. No longer can they impose annual or lifetime coverage limits. Pre-existing conditions are no longer a barrier to accessing coverage for oneself or one's children. And women cannot be charged higher premiums than men for the same plan. Additionally, insurers are now required to spend no more than 20 percent of consumers' premium dollars on overhead, and if they go over the limit, they are required to reimburse the difference.

Just last week, the Centers for Medicare and Medicaid Services released a report showing that the growth in health care spending nationally has continued to decline since the Affordable Care Act was enacted. In fact, the period from 2011-2013 saw the slowest rate of growth in health spending since 1960.

For Medicare beneficiaries, a range of preventative screenings and tests -- like annual wellness visits, mammograms, and colonoscopies -- are now free. The Part D prescription drug coverage "donut hole" is closing, and more than 8.2 million seniors have saved over $11.5 billion on their medications since the Affordable Care Act became law in 2010. There have been significant benefits to younger Americans too; since its enactment, the law has enabled as many as 3 million people under age 26 to remain covered under their parents' plans. From school days to the golden years, the Affordable Care Act is delivering benefits that are increasing access, raising quality, and controlling costs.

In the next Congress, Democrats will be working to strengthen the Affordable Care Act and ensure more Americans have access to quality, affordable health care. Instead of trying to repeal or undermine the law, Republicans ought to work with Democrats so people across our country can live longer and healthier lives.