AARP is targeting 11 GOP senators, including key centrists, to oppose the House-passed healthcare bill that would raise premiums for seniors.

The ad campaign expands a May effort that ran ads targeting five senators, calling for the House-passed American Health Care Act to be scrapped. The expansion comes at a pivotal time as Senate leadership hopes to vote on a healthcare bill by the end of July.

AARP is targeting Sens. Lisa Murkowski and Dan Sullivan of Alaska, Jeff Flake of Arizona, Cory Gardner of Colorado, Joni Ernest and Chuck Grassley of Iowa, Dean Heller of Nevada, Rob Portman of Ohio, Lamar Alexander and Bob Corker of Tennessee, and Shelley Moore Capito of West Virginia.

The list includes some key centrists who will be critical to the GOP leadership's hopes of passing its own version of the American Health Care Act before Congress' August recess.

Heller and Flake are up for re-election in 2018. Heller, Portman and Capito are pushing leadership for a seven-year phaseout of Obamacare's Medicaid expansion.

AARP, the nation's biggest seniors lobby, has been opposed to the American Health Care Act for some time, angry over a proposed change to premiums for senior citizens in insurance plans on the individual market.

Obamacare allowed insurers to charge seniors three times the amount they charge a younger person. The American Health Care Act would increase that to five times.

"Our members and other Americans over age 50 are very worried about legislation that would raise their premiums through what is, in effect, an age tax," said AARP Executive Vice President Nancy LeaMond.

It is not clear what pieces of the legislation the Senate will keep, including the age-rating ratio.

AARP also derided problems with Medicaid and hurting "protections for people with pre-existing conditions."

A controversial last-minute amendment to the legislation, which passed the House last month by a 217-213 vote, let states opt out of community rating mandate. States could get a waiver that would let insurers charge sicker people more money.

House Republicans say that $23 billion included in the legislation for high-risk pools could help offset any increases. A recent estimate from the Congressional Budget Office said that money wasn't enough to offset major increases for people with pre-existing conditions such as cancer or diabetes.