WASHINGTON — Overhauling the nation’s tax code in a once-in-a-generation opportunity, says House Speaker Paul Ryan, who is promising to act by the end of the year despite divisions among Republicans and a crowded legislative agenda for Congress.

The Wisconsin Republican and former chairman of the tax-writing Ways and Means Committee says the GOP, which controls Congress and the White House, has a chance to rewrite the tax code for businesses and individuals.

“We are going to get this done in 2017. We need to get this done in 2017. We cannot let this once-in-a-generation moment slip,” Ryan says in prepared remarks. “Transformational tax reform can be done, and we are moving forward. Full speed ahead.”

Ryan warned “against accepting half measures or believing there is a path to reform without obstacles and political challenges.”

That’s an apparent jab at a growing number of Republicans who simply want to cut taxes rather than simplify the tax system and make it more efficient.

Ryan’s office says he will make the case for permanent tax reform — rather than a temporary tax cut.

The obstacles in Congress are many.

Ryan supports a new tax on imports to help finance a lower overall tax rate for corporations, and to encourage U.S. companies to stay in the United States. The tax, however, has no support in the Senate and is vigorously opposed by retailers who worry that it will increase the cost of consumer goods.

Republicans are planning to pass a tax package under a procedure in which they need only a simple majority in the Senate — preventing Democrats from blocking it. Under the procedure, the tax package cannot add to long-term budget deficits.

That means for every tax cut, there has to be a tax increase, at least over the long term.

There’s more. To take advantage of the procedure, Republicans in the House and Senate must pass a budget resolution. That won’t be easy because there are big disagreements among Republicans over spending levels for the military and for domestic programs.