* Showdown votes in House and Senate this week
* Government shutdown, credit default at risk
By Thomas Ferraro and Rachelle Younglai
WASHINGTON, Sept 26 (Reuters) - U.S. House of Representatives Republicans on Thursday refused to give in to President Barack Obama's demands for straightforward bills to keep the government running beyond Sept. 30 and to increase borrowing authority to avoid a historic default.
In a direct challenge to Obama, they said they will seek a one-year delay in the full implementation of the new national healthcare law known as "Obamacare" in return for raising U.S. borrowing authority by enough to let Treasury borrow through the end of 2014.
Even as some senior Republicans predicted there would be no government shutdown on Oct. 1 or credit default next month, House Speaker John Boehner detailed a legislative path putting his chamber on a collision course with Obama and Senate Democrats.
Asked by a reporter whether the House would sign off on a government spending bill the Senate is expected to pass on Saturday, which simply extends current funding for another six weeks, Boehner replied: "I do not see that happening."
The top Republican made his remarks after a closed meeting with his rank and file.
With the current fiscal year ending in just five days and no laws in place to fund the government in the new year, House Republicans also discussed the possibility of widespread government agency shutdowns beginning on Tuesday.
"We did discuss contingency plans in case of a shutdown; what we have to do in terms of furloughs and continuing services," Representative John Fleming of Louisiana told reporters. He added that letting government funding run out, even temporarily, "is not a goal at all."
But it was still unclear whether the Republican-controlled House and the Democratic-controlled Senate could work out their differences in time.
House Republicans passed an emergency spending bill last week to defund Obamacare. But with Democrats standing firm against that tactic, Republicans have begun looking at other items their conservative members might attach to the bill.
Senate Democrats intend to pass a "clean" bill to keep the government running from Oct. 1 to Nov. 15 and send it to the House as the clock ticks down to the expiration of government funding at midnight on Monday.
Republican Representative Tom Cole of Oklahoma said there are discussions in the House about attaching a measure, which he did not specify, to the funding bill that has bipartisan support in the Senate.
One such measure could be the proposed repeal of a medical device tax that would collect $30 billion over 10 years and help pay for some of the costs of Obamacare.
Few Republican add-on measures, if any, are likely to be palatable to Democrats, but are aimed at pleasing conservative Tea Party activists who want to shrink government, even if it means provoking confrontations with the White House that are likely to fail.
Senator Charles Schumer of New York, the third-ranking Democrat in the Senate, said his party was "resolute" against any such add-ons to the emergency spending bill. Similarly, the White House declared that it would reject a medical device tax repeal in exchange for keeping the government running.
"We are not accepting riders. We want a clean CR," Schumer said referring to the government funding bill that is known as a "continuing resolution" or "CR."
House Republicans are also challenging Obama on the debt limit increase that the Treasury Department says is urgently needed by Oct. 17.
In an opening move, House Republican leaders said that in addition to seeking to delay Obamacare, they will also attach new spending cuts and other initiatives to a debt limit bill, something that Obama has said he would not tolerate.
White House spokesman Jay Carney on Thursday called the Republican tactic a "political extortion game."
Obama repeatedly has warned that he wants a debt limit increase with no strings attached.
Along those lines, the White House on Thursday said that it would not go along with a Republican proposal authorizing construction of the Keystone oil pipeline running from Canada to the U.S. Gulf of Mexico as part of a debt limit increase bill.