Republicans have made lower insurance premiums one of their biggest promises on healthcare.

But an analysis by the Kaiser Family Foundation found that once you factor in tax credits, the endpoint premium payment for most Americans in the individual health insurance market would increase under the Better Care Reconciliation Act, Senate Republicans' bill to overhaul the US healthcare system.

The nonpartisan health-policy think tank broke down just how much premiums for a silver-level plan in the individual insurance market would increase for people of different ages, factoring the effect of proposed tax credits.

While the individual market includes only about 7% of the population, the premiums in the market have become a central part of the debate over the future of healthcare. Republicans often bring up the increase in premiums in this market as a reason that the Affordable Care Act, or Obamacare, needs to be replaced.

Overall, Kaiser found that premiums after tax credits in the market would increase by 74% under the BCRA compared with the ACA.

Additionally, older Americans would see an even steeper increase for a variety of reasons.

For one, older people could be charged more under the BCRA than the ACA. Under current law, older Americans can be charged only three times as much for premiums as younger Americans. Under the Senate bill, this would increase to five times as much.

Additionally, the premium tax credit structure would be changed. The formula for how much of a person's income is supposed to be spent on a premium would gradually increase as they age under the BCRA.

Add it all up, and almost everyone would see an increase in their premiums for a silver plan, becoming more dramatic the older someone gets.

Given the premium increases, there is also a good chance that people would end up getting less-generous health insurance.

According to an analysis by the nonpartisan Congressional Budget Office, this kind of jump would mean that some people on the individual market who currently have a plan in the silver bracket or higher might trade down to a bronze plan, which has less-generous coverage. Currently, under a silver plan, 70% of healthcare costs are covered by the insurer; in a bronze plan under the BCRA, that figure would be 58%.

Insurers paying a lower percentage of costs means people would have to make up the difference, leading to higher deductibles and co-pays. So to get affordable premiums, people would have to get plans with higher out-of-pocket costs.

According to the Kaiser study, the situation could be worse for people with lower incomes, who would see much less generous tax credits for the same silver plan.

While the raw dollar jump for those making above 200% of the federal poverty line would be dramatic, the percentage increase for low-income people would be worse. According to Kaiser, the premium increase for someone between 55 and 64 with an income above 200% of the federal poverty line would be 96%, to $782 a month from $399. For someone in the same age bracket making less than 200% of the poverty line, however, it would be a whopping 294% jump, to $272 from $69.

If the bill were to become law, regardless of how much people make or how old they are, it wouldn't be a question of whether premiums after tax credits would increase, but by how much.