President Obama’s healthcare law won’t erode employer-based health insurance — but it will raise some companies’ costs by nearly 10 percent, according to a new analysis from the Urban Institute.

Although the law’s critics usually focus on small businesses, the new paper says medium-sized firms will see the biggest cost increase.

Mid-sized businesses — firms that have between 101 and 1,000 employees — would have seen a 9.5 percent jump in their total healthcare costs if the Affordable Care Act had been fully in place this year, the paper says. (Many of the law’s key provisions don’t take effect until 2014.)

Small businesses would have seen their costs fall by 1.4 percent. Firms with more than 1,000 workers would have seen a 4.3 percent increase.