I'm not an expert at using Bitcoin core, and this link from 2013 might contain out of date information.
https://bitcointalk.org/index.php?topic=232979.0However, it says core will rebroadcast unconfirmed transactions every 30 minutes while the sender keeps his wallet open. If you read the whole thread it explains how to deal with unconfirmed transactions, but as it's from 2013 it might be out of date. I'm sure other users here can advise you better than me about core.
How do I resend a transaction?
Bitcoin-qt will rebroadcast unconfirmed transactions every 30 minutes or so.
As long as the sender has his client open, the transaction will be rebroadcast.How do I resend a transaction with extra fee?
For Bitcoin Core users:
- Right click the transaction and choose "Abandon Transaction"
- Send the coins again with a fee
Third party client usersHow do I remove an unconfirmed transaction?
If you do not want to see the unconfirmed transaction, you can
remove the transaction from your wallet. Once the transaction gets confirmed, it will reappear in your wallet.