PORTLAND — For weeks now, Republican Sen. Gordon Smith has been hammering his Democratic challenger, Oregon House Speaker Jeff Merkley, in television ads that imply Merkley has never met a tax hike he didn’t like.

On Wednesday, Merkley struck back with his own lengthy list of tax and fee increases that Smith had voted for, noted his own votes for tax credits and cuts in the Oregon Legislature, and outlined an $88.7 billion proposal aimed at cutting middle-class taxes.

The early back-and-forth suggests that the debate over taxes, spending and the economy could be a dominant theme in this fall’s campaign.

Smith has been staking out moderate positions on social issues. But he is generally considered to be a reliable Republican vote on tax issues, and has said he thinks the deep tax cuts put in place by the Bush administration should be made permanent.

On Wednesday, the Merkley campaign criticized that position, saying the Bush administration cuts primarily benefit the wealthy — though most middle-class families also pay less than they did in the 1990s. Rolling those cuts back would pay for a host of middle-class tax credits, Merkley said, including:

n A $500 tax credit for nearly every worker, and the elimination of income taxes for seniors who make less than $50,000, an idea that’s also been floated by Democratic presidential nominee Barack Obama.

n A $4,000 tuition tax credit, via an expansion of current tax law.

n A $1,500 tax credit for first-time homebuyers.

He’s also proposed increasing existing credits for child care costs and providing care for aging parents.

The package seems carefully crafted to combat the image of Merkley that’s been running in Smith’s television ads, a clip of Merkley claiming that he advocates for tax hikes “every night,” — though the clip does not include the rest of Merkley’s statement, in which he specifies that he means closing tax loopholes for large corporations.

The Smith campaign also has  dinged Merkley for voting to support some tax proposals in the Legislature, though all the proposals they cite came at a time when the Oregon House was under Republican control, and thus had enough support from the GOP to make it to a floor vote.

Though the Merkley campaign released a list of tax hikes and fee increases that they said had gotten Smith’s OK over the years, Lindsay Gilbride, a Smith spokeswoman, dismissed that as a campaign ploy, saying many of them were instances where, “it would be taxing Peter to pay Paul.” For example, she said, the Merkley campaign’s claim that Smith voted to continue higher taxes for married couples was on a bill that would have reduced marital taxes only for double-income families.

She also said Smith is working on some of the same tax credit proposals Merkley is pushing, including making the child care tax credit refundable and supporting a $7,500 credit for first-time homebuyers.